In the electronics industry, a substrate or board is used to support integrated circuits or electronic components. The circuitry connecting these components is often provided by applying electrically conductive lines, such as by printing, vapor deposition or other means, on one or more surfaces of these substrates. In many applications, it is advantageous to form these substrates substantially from ceramic materials because such ceramic materials can be fashioned so as to possess desired electrical insulating characteristics, as well as good thermal or chemical stability.
The recent history of the electronics industry has shown the desirability, in many applications, of miniaturization, i.e. placing a larger number of components and circuit elements onto a smaller surface area. In connection with this trend, it is often advantageous to provide the surface of a substrate with electrical circuitry which is extremely fine-lined.
Experience has shown that in order for fine-lined circuitry to be placed on a surface of the substrate, the substrate must be substantially without surface defects such as pits, voids, protrusions, and so on. The degree of smoothness necessary for a substrate depends upon the fineness of the desired lines, i.e. the minimum width of the circuitry lines to be placed on the surface. Generally finer circuitry lines require a greater degree of surface smoothness.
Several methods have been used in an attempt to provide the required surface smoothness for a ceramic substrate. Polishing a substrate surface is unacceptably costly because of the hardness of ceramic materials and furthermore is effective only to remove protrusions and cannot satisfactorily fill voids or pits. Glazing the substrate with a thin layer of a material such as a glass produces undesirable characteristics such as reductions in adhesion, thermal conductivity, chemical resistance and electrical insulation between adjacent conductive paths. Spraying or otherwise placing a layer of finegrained flowable ceramic material on a surface of a dry pressed product or body suffices for some purposes, but has its own drawbacks. This technique depends on a "reflow" of the sprayed ceramic slurry to establish a flat surface by virtue of quasi-fluid flow under the influence of gravity. Thus, surface characteristics such as smoothness can be adversely affected by surface tension, by the viscosity of the slurry, and other factors. This method results in covering substantially the entire surface of the dry pressed product and thus is not useful when it is desired to provide a "pull back" in which the high quality smooth surface is set back some distance from the edges of the dry pressed product. When the finished product or ceramic substrate includes one or more holes extending through the substrate, spraying a fine-grained ceramic slurry on the dry pressed product may result in the slurry partially flowing over the holes of the dry pressed product, in some cases causing parts to fail to meet pin or metric eye tests. Finally, the spray method normally requires two firings of the substrate, one firing before the spraying operation and a second firing after the spraying operation. This two-firing process increases substrate expense compared with methods requiring a single firing.